(1) Field of the Invention
The present invention relates to an output level automatic control (ALC) apparatus suitable for use to keep a transmitted output level in a transmitting board in a radio system such as a multiplex radio system or the like constant.
(2) Description of the Related Art
A digital multiplex radio system is, in general, required to keep an output level of a transmitter constant by absorbing fluctuations in output power due to temperature fluctuations, power-source voltage fluctuations, deterioration with age and the like. Therefore, the digital multiplex radio system is generally provided with an AIC apparatus where a part of a transmitted output is taken out by a directional coupler or the like to be detected and compared with a reference voltage to be amplified so that an attenuation quantity of a variable attenuator is regulated with that voltage. When an output level of the transmitter decreases, this ALC apparatus decreases an attenuation quantity of the variable attenuator, that is, a negative feed-back control is performed.
FIG. 11 is a block diagram of a radio transmitting unit having an ALC apparatus. As shown in FIG. 11, the radio transmitting unit has a modulating unit 17, a preamplifier 18, a variable attenuator 12, an up-converter 19 (including a mixer 19A and a local oscillator 19B), a high-output amplifier (a main amplifier or a final stage amplifier) 11 and an antenna 20, in the order arranged from an input side of the radio transmitting unit on a transmission line of the same.
The ALC also has a feed-back control loop (a main amplifier output level feed-back control loop) 13 extending from an output side of the high-power amplifier 11 to the variable attenuator 12. The feed-back control loop 13 includes a directional coupler 14A and a detector 14B, as an output level detecting means, a reference value setting unit 15 setting a reference value and a control unit 16A.
The directional coupler 14A takes out an output level of the high-output amplifier 11 to a side of the feed-back control loop 13, while the detector 14B detects the output level of the high-output amplifier 11 taken out by the directional coupler 14A by detection.
The control unit 16A outputs a control signal used to control the output level of the high-output amplifier 11 to the variable attenuator 12 on the basis of a result obtained by comparing the output level of the high-output amplifier 11 detected by the detector 14B with the reference value set by the reference value setting unit 15, including a comparison amplifier.
The control unit 16A has a control characteristic shown in FIG. 13, where when the output level decreases, that is, an input voltage Vdet to the control unit 16A decreases, an output voltage (control voltage) Vcnt outputted from the control unit 16A is increased to decrease an attenuation quantity of the variable attenuator 12. In other words, the control unit 16A has a control characteristic to perform a negative feed-back control (that is, a characteristic monotonously descending towards the right) when the output level, that is, the input voltage Vdet decreases.
Such control characteristic of the control unit 16A is accomplished by that the variable attenuator 12 is so configured that the attenuation quantity of the variable attenuator 12 is decreased as the control voltage (output voltage) Vcnt is increased, and that the detector 14 is so configured that the detected voltage (input voltage) Vdet is decreased as the transmitted output level Po is decreased.
Therefore, the control unit 16A may have an inverting amplifier U3 as shown in FIG. 14. The amplifier U3 is a linear amplifier, using an operational amplifier (an OP amplifier) or the like. The control voltage Vcnt and the detected voltage Vdet are equal to 0 V or more. Incidentally, R101, R102 and R103 shown in FIG. 14 are resistors.
In FIG. 11, when the transmitted output level Po decreases, the detected voltage Vdet decreases as well, and consequently the output voltage Vcnt of the control unit 16A increases. As a result, an attenuation quantity of the variable attenuator 12 decreases and the transmitter output level Po increases, becoming stable at a setting value.
FIG. 12 shows another proposal, where the variable attenuator 12 is interposed between the up-converter 19 and the high-output amplifier 11. In this case, the ALC operates in the same manner as the above-mentioned case shown in FIG. 11.
However, such conventional ALC apparatus so operate as to decrease an attenuation quantity of the variable attenuator 12 when an output level Po of the transmitter decreases. It means that when an input signal is not inputted to the transmitter, the transmitter operates at a maximum gain. If a signal at a level of a normal operational state is inputted under such condition, the output level Po exceeds a specified value until the ALC apparatus responds. In the worst case, the high-output amplifier 11 as a final stage amplifier operates in an overinput state, which might cause deterioration of amplifying elements such as an FET and the like forming the high-output amplifier 11.
It may be considered that the same phenomenon occurs upon on/off of a power source. Therefore, the same problem will arise in this case.
To overcome the above problem, there have been proposed an apparatus in which a control unit configured with a digital circuit including an A/D converter, a microprosessor, an A/D converter, etc. makes a judgement of "no output power" by watching an output level to keep an attenuation quantity of an variable attenuator in a state immediately before "no output power" refer to Japanese Patent Laid-Open (Kokai) Publication No. 63-81515!, or an apparatus in which a control unit makes a judgement of "no input power" by watching an input level so as to make a variable attenuator operate at a maximum attenuation quantity when there is "no input power" refer to Japanese Patent Laid-Open (Kokai) Publication No. 2-1041311!.
Above conventional ALC apparatus require a digital circuit to make a judgement of "no output power" or "no input power", an input level watching circuit and the like, causing an increase in size of the circuit or an increase in cost.